The present invention relates to a sensor for the contactless linear measurement by backscattered radiation of the distance separating the sensor from an optionally moving target.
The prior art already discloses sensors for the linear measurement of such a distance, such as those e.g. described in the article entitled "Use of Optical Proximity Sensors in Industrial Robotics", pp 47 ff of the journal "Le Nouvel Automatisme", May/June 1980 or in French patent application No. 83 17 183 of 27.10.1983 entitled "Backscattered radiation proximeter usable in robotic telemanipulation and associated processing system", now French Pat. No. 2554244.
Most devices of this type operate by using sensors formed from a light radiation transmitter or emitter and a receiver, separated by a distance d from one another, whereby the target whose distance d is to be measured is used for reflecting a light beam from the emitter and which is reflected towards the receiver.
A known device of this type is very diagrammatically shown in FIG. 1, where it is possible to see a light emitter E and a receiver R spaced by .delta. and the surface S of a target moving with respect to the sensor, the distance between surface S and the sensor being equal to d. A light beam emitted by emitter E in the direction of point M of surface S is assumed to form an angle .theta. with the normal MN to the surface S at point M. It is reflected in accordance with the previleged direction .theta. towards towards receiver R. There is a certain dispersion of the reflected light, but the maximum intensity of the backscattered beam is, according to the laws of optics, in angular direction .theta. symmetrical of the incidence direction with respect to the normal MN.
The device of FIG. 1 implies that emitter E and receiver R are inclined by an angle .theta. with respect to the normal to the support.
On studying within a system of this type, the light intensity I received by receiver R from the sensor as a function of the distance d separating the sensor from the target surface S, a curve 1 is obtained having the general configuration shown in FIG. 2. Curve 1, starting from zero, develops to a maximum for distance d=d.sub.0 and then decreases asymptotically to a zero value when distance d tends towards infinity. In other words, for a given geometrical constitution of the assembly of FIG. 1, there is a distance d.sub.0 from the sensor to the target for which the intensity of the signal received by receiver R is maximum. On modifying the physical properties of surface S of the target, it is possible to obtain other curves, such as e.g. curve 2, whereof the maximum is located at a lower amplitude, but all the maxima of the different curves are for the same value d.sub.0 of distance d. Hereinafter it will be noted that it is possible to divide the path of graphs 1 and 2 representing the laws I=F(d) into three portions, namely a first portion d=0 to d=d.sub.1 in which the graph representing the function I=F(d) is substantially a straight line and consequently law I=F(d) is linear; a second portion extending from value d.sub.1 to value d.sub.2 of d, i.e. in the vicinity of the maximum M of each of the graphs 1 and 2, where the preceding curves are substantially quadratic, i.e. they could be represented by a parabolic development, whose mathematical expression would be close to I=K.sub.1 +K.sub.2 d+K.sub.3 d.sup.2 ; finally between value d=d.sub.2 and infinity, a third part where the function I=F(d) is substantially decreasing parabolic and can be mathematically represented in simplified form by a function of type I=k/Vd.
In other hitherto distance measuring sensors or optical proximeters, use is solely made of the first linear part of the aforementioned graphs, i.e. that separating the zero distance from distance d=d.sub.1, so as to be able to have a sensor expressing the result of the sought measurement by an analog magnitude substantially proportional to the distance which it is wished to measure. Such sensors function correctly, but it is clear that the limitation imposed in this case for the measuring range with respect to the graphs of FIG. 2 constitutes a serious limitation to the range of application of such equipment, particularly for values exceeding the distance d.
The present invention relates to a sensor for the contactfree linear measurement by backscattered radiation of the distance d separating the sensor from a possibly moving target and which permits the use both of the first and third parts of the graphs of FIG. 2. In other words, this sensor operates by solely removing the parabolic zone located in the case of the graphs of FIG. 2 in the immediate vicinity of the maximum M thereof. Thus, this leads to a better definition and a greater accuracy of the sought result.